This research is a continuation of earlier studies of cell-mediated immunity in avian leukosis. In previous studies a cytotoxic assay system for cell-mediated immunity based upon the release of Cr51 label from chicken red blood cell targets was developed. In this continuation study, the cytotoxic assay will be further characterized and adapted to nucleated somatic cell targets. Splenic and peripheral lymphoid cells will be used in the test and adaptation of the test to a microtiter system will be undertaken. The influence of age, sex genetic makeup and infection status with lymphoid leukosis virus will be studied. The cytotoxic assay system will be compared as a correlate of cell-mediated immunity to the capillary tube migration inhibition test.